Can't (Always) Drive 55: How Fast You Can Go on I-76 Will Soon Be Variable Dan Stamm What to Know The speed limit on the Schuylkill Expressway between King of Prussia, Montgomery County, and Philadelphia soon will be changing depending on conditions. PennDOT on Thursday marked the activation of variable speed limit signs. Around 130,000 drivers use the 14-mile stretch of I-76 daily, PennDOT says. The speed limit could soon be changing as you drive along the crowded Schuylkill Expressway. PennDOT officials marked the activation of 72 variable speed limit (VSL) signs over a 14-mile stretch of Interstate 76 from King of Prussia, Montgomery County, to the U.S. Route 1 Interchange in Philadelphia.